1. Field of the Invention
The invention concerns a method for generation of a homogeneous magnetization in a spatial examination volume of a magnetic resonance system during a subject examination, the magnetic resonance system having a body coil formed by a number of resonator segments and a control and evaluation device for separate activation, corresponding to a set of predetermined, segment-specific excitation parameters, of the individual resonator segments, that are electromagnetically decoupled from one another.
2. Description of the Prior Art
Magnetic resonance tomography is one of the imaging methods in medical diagnostics that utilizes the interaction of an external field (here a magnetic field) with the human body for imaging. The design and the functionality of such a magnetic resonance system is known and does not have to be described in detail.
In earlier times, magnetic resonance systems were also produced in which the basic magnetic field was more than 1.5 T, in particular 3 T and more. Better resolutions can be achieved with these magnetic resonance systems, but inhomogeneities of the image quality occur that are ascribed to inhomogeneities in the excitation field generated by the body coil.
A magnetic resonance system is known from EP 1 279 968 A2 in which, according to the system design described above, resonator segments that are electromagnetically decoupled from one another are provided that form the body coil. Here the individual resonator segments can resonate independently of one another at the desired resonance frequency. A separate transmission channel is associated with each resonator segment, meaning that each resonator segment can be activated separately. The generation of separate individual fields that, in combination, produce the circularly-polarized radio-frequency magnetic field, is possible. The amplitude and the phase of each individual transmission channel can be adjusted correspondingly.
As described, the image quality in magnetic resonance systems depends to a considerable degree on the homogeneity of the spatial distribution of the component of the circularly-polarized radio-frequency magnetic field that effects the deflection of the spins from the equilibrium state. Especially at higher frequencies, the homogeneity of the magnetic field is no longer significantly influenced solely by the currents at the antennas, but also by the currents in the patient's body. The activation of the resonator segments ensues using an established excitation parameter set that, with regard to the amplitudes and phases of the activation signals of the individual resonator segments, is selected to obtain an optimally homogeneous magnetic field.
A central problem in the framework of the MR examination with a comparably strong basic magnetic field is the patient exposure that exists due to the very high operating frequencies of the excitation magnetic field, which patient exposure is caused by power losses absorbed by the patient. For example, if a basic magnetic field of 3 T is used, the frequency of the circularly-polarized excitation magnetic field is, for example, 128 MHz. More severe inhomogeneities result from the high frequency and the low penetration depth of the magnetic field associated therewith, which severe inhomogeneities lead to the situation that the spin flip across the examination volume is not sufficient. However, a more severe heating of the body additionally results therefrom due to the high energy application. This increased energy application can not be arbitrarily high locally with regard to the examination volume nor globally with regard to the examination subject. In the MR examination, limit values are taken into account in order to avoid over-exposing the patient. The local specific absorption rate (SAR) concerns the local energy application per weight unit and is specified as a power loss density per kilogram of weight while the global absorption rate represents the sum of the local absorption rates over the entire examination subject. For example, the global absorption rate can lie below the limit value while the local absorption rate exceeds the limit value at a specific point of the examination volume and a local burning of the patient can occur.
The goal of an optimally homogeneous excitation magnetic field thus is in conflict with the problem of the patient exposure that limits the acquisition possibilities given excitation with a predetermined excitation parameter set that would lead to a very homogeneous magnetic field.